Add native mac copy/paste support to match win32 (#2546)

ImFont: Minor adjustment to the structure. ...

Examples: Removed unused variable.

ImDrawData: Added FramebufferScale field (currently a copy of the value from io.DisplayFramebufferScale). ...

This is to allow render functions being written without pulling any data from ImGuiIO, allowing incoming multi-viewport feature to behave on Retina display and with multiple displays. If you are not using a custom binding, please update your render function code ahead of time, and use draw_data->FramebufferScale instead of io.DisplayFramebufferScale. (#2306, #1676)
Examples: Metal, OpenGL2, OpenGL3: Fixed offsetting of clipping rectangle with ImDrawData::DisplayPos != (0,0) when the display frame-buffer scale scale is not (1,1). While this doesn't make a difference when using master branch, this is effectively fixing support for multi-viewport with Mac Retina Displays on those examples. (#2306) Also using ImDrawData::FramebufferScale instead of io.DisplayFramebufferScale.
Examples: Clarified the use the ImDrawData::DisplayPos to offset clipping rectangles.

Removed trailing spaces (#2038, #2299)

Examples: Downgrading projects to xcode 9.2 (maybe 8.0) (#2134) ...

* example_apple_opengl2: The deployment target was set to 10.12 from XCode 9.2.
* imgui_impl_metal: header not found by XCode 9.2.
* example_apple_metal: The deployment target was set to 10.12 from XCode 9.2.

Examples: Setting up style before bindings, so in complex binding (vulkan/dx12) it isn't miles away from the context creation.

Examples: Misc comments mainly related to GLFW callbacks. (#1759)

Examples: Downgraded Xcode project object version (50->48) to support Xcode 9.2 (last version supported on macOS 10.12) (#2133, #1929)

Examples: Using "dear imgui" terminology in all examples headers/comments + fix minor typo.

OSX: Added imgui_widgets.cpp to xcode project files.

Examples: Using fully qualified "Dear ImGui" name in window titles.

Examples: Tweaked the main.cpp example structure for all examples. (There are a few hidden agendas here: 1) I would like to avoid encouraging people from using the implicit "Debug" window, and promote using Begin/End. In spite of my best attempt, there are a few feature of the upcoming docking system that cannot work 100% properly for the implicit Debug window, so future proof let's not put that feature in the spotlight too much for new users. 2) Moved dumb hardcoded positions into a single spot that can be replaced with a viewport relative position and not affect other demo windows. 3) Calling ShowDemoWindow before anything else, also for the benefit of a specific docking demo which will have an ordering constraint which is not really problematic in a real app but shouldn't be put forward in the demo.

Apple: Fixed example_osx_opengl2, renamed to example_apple_opengl2 + misc comments